[Design View / Design Solution]
Zero-Drift IA Takes The Strain Out Of Sensor Measurements
Instrumentation amplifiers can abet many sensor applications, from ratiometric bridges to low-side current sensing.
APPLICATIONS This section describes two IA applications: a ratiometric bridge circuit and a low-side current-sense amplifier.
1. BRIDGE OVER TROUBLED WATERS A variation of the standard bridgemeasurement system is the ratiometric bridge, which delivers similar high accuracy but at a lower cost. Cost is lower because the ratiometric bridge doesn’t require a precision reference source for driving the bridge and ADC reference input. Instead, a “free” but relatively inaccurate and high-ppm/°C reference source, such as the power-supply rail, can be used to drive both the bridge and the ADC.
It’s well known that even an op amp with “rail-to-rail” output has trouble maintaining full accuracy while driving its output to within a few hundred millivolts of either rail. For an amplifier with high dynamic range and unipolar-signal inputs, it’s therefore necessary to bias the output above ground, by 250 mV or so. This bias voltage drives one end of a resistor chain and thus should be driven by a buffer of low output impedance to avoid introducing unintentional gain errors. To minimize output errors, this unity-gain op-amp buffer should also have low dc offset and low drift.
An IA from Maxim (the MAX4208) integrates a precision, zero-drift op-amp buffer with a 2-gm indirect current-feedback IA in a small µMAX package. This buffer allows a simple external resistor divider to be used to create a stable bias reference voltage that’s ratiometric with the ADC reference voltage. It’s also able to drive one of the inputs of a differential input ADC. The internal chopperchopper- stabilized architecture of the IA eliminates pink-noise effects in both the op-amp buffer and the gm amplifiers of the main (forward) and feedback paths. In addition, the part includes a shutdown mode that’s useful for powersensitive applications.
2. MAKE PERFECT CURRENT-SENSE The increasing need for active power management in today’s portable electronic devices has led to a renewal of interest in current-sensing amplifiers. A ground-sensing IA can be used as a high-side current-sense amplifier in the core-voltage path of a memory module or microprocessor (Fig. 4) or as a low-side current-sense amplifier in the return path of an H-bridge power electronic converter.
The extremely high currents in these applications (sometimes approaching 90 A) imply that the sense voltage must be extremely small to prevent excessive power loss in the sense resistor. Quite often, this sense resistor is simply the ESR of the power-supply inductor itself. To read these small sense voltages accurately, the input offset voltage must be extremely small in comparison with smallest sense voltage (i.e., smallest load current) that’s required to be amplified with accuracy.
Core voltages in computer hardware can vary from 0.9 to 1.5 V, and so the small sense voltage must be measured in the presence of a low and varying common-mode voltage. An IA such as the MAX4208 with low VOS, high CMRR, and an architecture optimized for single- supply applications is thus ideal for this purpose.
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